Use a Sum or Difference Identity to Find the Exact $\cos 36 ^ { \circ } \cos 24 ^ { \circ } - \sin 36 ^ { \circ } \sin 24 ^ { \circ }$

Multiple Choice

Use a sum or difference identity to find the exact value.
- $\cos 36 ^ { \circ } \cos 24 ^ { \circ } - \sin 36 ^ { \circ } \sin 24 ^ { \circ }$

A) $\frac { \sqrt { 3 } } { 2 }$
B) $\frac { \sqrt { 2 } } { 2 }$
C) 1
D) $\frac { 1 } { 2 }$